1. Field of the Invention
The present invention relates to an acoustoelectric converter element, particularly to an acoustoelectric converter element which optically detects mechanical vibration due to an acoustic wave, thereby to convert the vibration into an electrical signal.
2. Description of the Related Art
As an acoustoelectric converter element which converts mechanical displacement based on an acoustic wave into an electrical signal, a microphone is known. A general microphone has a structure in which a diaphragm mechanically vibrated by the acoustic wave is incorporated, and the resonant frequency is uniquely determined by the mass and spring constant of the diaphragm. The frequency characteristic is said to be nominally flat, but sensitivity tends to drop apart from the resonant frequency inherent in the diaphragm. As a method of realizing a microphone having a broad dynamic range, a method in which a plurality of diaphragms having different diaphragm sizes and spring constants are arranged is reported in Jpn. Pat. Appln. KOKAI Publication Nos. 2001-292498 and 2001-231100.
On the other hand, there has been known a condenser microphone in which the vibration displacement of the diaphragm is generated due to a sound pressure and is detected as capacitance change. There is also known an optical microphone in which a light wave is irradiated on a diaphragm vibrating due to the sound pressure and a change of light intensity of the reflected light wave is detected as the vibration displacement. The condenser microphone and the optical microphone are disclosed in the Jpn. Pat. Appln. KOKAI Publication Nos. 2001-292498 and 2001-231100. In the optical microphone, superior characteristics are expected from viewpoints of directivity and resistance to noises as compared with the condenser microphone.
In a conventional optical microphone, a laser wave of gas laser, solid laser, or semiconductor laser diode is projected on the diaphragm and a change of light intensity of the reflected light wave is detected as the vibration displacement of the diaphragm. In the optical microphone which optically detects vibration amplitude of the diaphragm in this manner, an electrode or the like is not necessarily disposed on the back surface of the diaphragm. Therefore, the same space situation can be produced on front/back of the diaphragm, and therefore a sound pressure inclination microphone can be formed.
Moreover, there is a microphone array technique in which a plurality of condenser microphones are used, a delay sum is utilized, and accordingly single directivity is realized. A method is also used in which a pipe is additionally disposed in a part of an originally sealed vessel to supply outside air behind the diaphragm, the acoustic wave from the front is transmitted to the back surface of the diaphragm with delay to impart a “speed type” operation, and an operation of “single directivity” is performed as a whole. In the method proposed in the Jpn. Pat. Appln. KOKAI Publication Nos. 2001-292498 and 2001-231100, there is a disadvantage by a restriction of a mechanical structure or dimension. Since product groups increase in actual mounting, a drop of yield is feared.
Moreover, the conventional optical microphone disclosed in the Jpn. Pat. Appln. KOKAI Publication No. 2001-292498 is a reflective microphone. In the structure, high optical precision is required in an incidence optical system of incident light upon a diaphragm acting as the diaphragm and a reflection optical system which guides reflected light into a predetermined photo detector. Therefore, an optical fiber and a light guide have been used in order that the incident light and reflected light take arbitrary optical paths. Moreover, a lens device is disposed between a substrate on which a light receiving emitting device is mounted and the diaphragm, or another measure is taken in order to largely increase a movement width of the reflected light corresponding to the vibration displacement of the diaphragm. That is, in a conventional convergence grating system of the optical microphone, since the light wave reflected from the diaphragm spreads, great light intensity for detecting micro vibration displacement of the diaphragm by the photo detector to detect a difference of the light intensity is required, and therefore an assisting or correcting element for the optical path is required. Therefore, an optical device which realizes sophisticated positioning, and a new system are required in addition to the light emitting and receiving device and the diaphragm, and high-precision optical alignment is required. Therefore, there is possibility that drop of yield of a product is caused. Accordingly, a cost of the system rises. In a case where the optical fiber or the light guide is used, there is a problem that use range and application are largely limited. As understood from the structure, the light reflecting microphone has a problem that miniaturization is difficult.
The optical microphone capable of realizing sharp directivity with a simple structure is very useful, light source, diaphragm, photo detecting device need to be disposed in strictly appropriate positions, and difficulty at an assembling time is involved. Since the above-described three devices are individually introduced into the conventional optical microphone, a large space is wasted for the alignment of the devices. There is a great demand for arrangement in a limited small space in a microphone system. It can be easily speculated that it is difficult to adapt the existing optical microphone to the system, because the microphone has to take a large two or three-dimensional space.